Add --test-on-replica-manual-replication-control flag

This will wait indefinitely for the replication status to change.
This allows us to run test schema changes in RDS without needing
custom RDS commands in gh-ost.

go/base/context.go

@@ -85,6 +85,7 @@ type MigrationContext struct {
 	Noop                     bool
 	TestOnReplica            bool
 	MigrateOnReplica         bool
+	ManualReplicationControl bool
 	OkToDropTable            bool
 	InitiallyDropOldTable    bool
 	InitiallyDropGhostTable  bool

go/cmd/gh-ost/main.go

@@ -60,6 +60,7 @@ func main() {
 	flag.BoolVar(&migrationContext.SkipRenamedColumns, "skip-renamed-columns", false, "in case your `ALTER` statement renames columns, gh-ost will note that and offer its interpretation of the rename. By default gh-ost does not proceed to execute. This flag tells gh-ost to skip the renamed columns, i.e. to treat what gh-ost thinks are renamed columns as unrelated columns. NOTE: you may lose column data")
 
 	executeFlag := flag.Bool("execute", false, "actually execute the alter & migrate the table. Default is noop: do some tests and exit")
+	testOnReplicaWithManualReplicationControl := flag.Bool("test-on-replica-manual-replication-control", false, "Same as --test-on-replica, but waits for replication to be stopped, instead of stopping it automatically. (Useful in RDS.)")
 	flag.BoolVar(&migrationContext.TestOnReplica, "test-on-replica", false, "Have the migration run on a replica, not on the master. At the end of migration replication is stopped, and tables are swapped and immediately swap-revert. Replication remains stopped and you can compare the two tables for building trust")
 	flag.BoolVar(&migrationContext.MigrateOnReplica, "migrate-on-replica", false, "Have the migration run on a replica, not on the master. This will do the full migration on the replica including cut-over (as opposed to --test-on-replica)")
 
@@ -149,6 +150,13 @@ func main() {
 	if migrationContext.SwitchToRowBinlogFormat && migrationContext.AssumeRBR {
 		log.Fatalf("--switch-to-rbr and --assume-rbr are mutually exclusive")
 	}
+	if *testOnReplicaWithManualReplicationControl {
+		if migrationContext.TestOnReplica {
+			log.Fatalf("--test-on-replica-manual-replication-control and --test-on-replica are mutually exclusive")
+		}
+		migrationContext.TestOnReplica = true
+		migrationContext.ManualReplicationControl = true
+	}
 	switch *cutOver {
 	case "atomic", "default", "":
 		migrationContext.CutOverType = base.CutOverAtomic

go/logic/applier.go

@@ -566,14 +566,41 @@ func (this *Applier) StartSlaveSQLThread() error {
 	return nil
 }
 
+func (this *Applier) isReplicationStopped() bool {
+	query := `show slave status`
+	replicationStopped := false
+
+	err := sqlutils.QueryRowsMap(this.db, query, func(rowMap sqlutils.RowMap) error {
+		replicationStopped = rowMap["Slave_IO_Running"].String == "No" && rowMap["Slave_SQL_Running"].String == "No"
+		return nil
+	})
+
+	if err != nil {
+		return false
+	}
+	return replicationStopped
+}
+
 // StopReplication is used by `--test-on-replica` and stops replication.
 func (this *Applier) StopReplication() error {
+	if this.migrationContext.ManualReplicationControl {
+		for {
+			log.Info("Waiting for replication to stop...")
+			if this.isReplicationStopped() {
+				log.Info("Replication stopped.")
+				break
+			}
+			time.Sleep(5 * time.Second)
+		}
+	} else {
 		if err := this.StopSlaveIOThread(); err != nil {
 			return err
 		}
 		if err := this.StopSlaveSQLThread(); err != nil {
 			return err
 		}
+	}
+
 	readBinlogCoordinates, executeBinlogCoordinates, err := mysql.GetReplicationBinlogCoordinates(this.db)
 	if err != nil {
 		return err